I apologized, but he told me that apologizing wouldn’t end the problem, and because of that, I slapped him in the face.
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means come to an end so the verb vocab version is to come to an end.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you: Your romp finally has to come to an end when someone decides to sue (す) you for all your romping.